1. Field of the Invention
The present invention relates to differential signal circuits, and in particular, to circuits and methods for detecting differential signals with compensation for variations in circuit manufacturing processes, power supply voltage and operating temperature.
2. Related Art
Many systems, particularly differential signal systems, need to detect the amplitude of the input signal and compare it to the reference signal. By determining if the amplitude of the input signal is greater than or less than the predetermined reference, it can be determined whether an input signal is present. If no signal is present, remaining portions of the system can be idled or shut down to save power. Further, certain circuit operations need only be started upon detection of an input signal.
As is well-known in the art, and discussed in more detail below, the basic stages of subject signal detection circuit are an input stage, a rectification stage (also discussed hereinbelow as a detection stage), a reference voltage generator stage and a comparator stage. The comparator compares the output of the rectification stage to the predetermined reference, e.g., a reference voltage. The input signal magnitude causing the comparator to change its output signal state is the trip point of the system and should, typically, equal the input signal magnitude sought to be detected.
As is well-known, the output signal of the rectification stage and a reference voltage generator stage will vary over process (e.g., manufacturing process), power supply voltage and temperature (PVT). Additionally, the input stage often shows significant performance variations over PVT, and a significant contributor to these variations is the transconductance of the input stage devices. Accordingly, to correctly determine if the input signal is greater or less than the reference, the signal path (e.g., the input and rectification stages) and reference voltage generator stage need to track one another in order to minimize variations in the trip point as over PVT varies.
One conventional technique to achieve such tracking has been done by sending the reference signals through a dummy signal path implemented as a replica of the input and rectification stages so that the input signal and reference signal see the same path. However, this consumes additional circuit area and power.
Referring to FIG. 1, one example of a conventional rectification stage 10 can be implemented as shown. The differential input signal phases VINP, VINN are applied to the differential input amplifier formed by bipolar junction transistors Q1, Q2. Another transistor Q3 is part of a common mode circuit implemented as a voltage follower circuit driven by a common mode voltage VCM (discussed in more detail below). Current sources 12, 14 provide equal biasing currents I. This results in load currents I1, I2 being conducted by the load resistances RL, producing the positive OUTP and negative OUTN phases of the differential output signal. The rectification occurs due to the difference in voltages at the transistor emitters and the taking of the output at the collector electrodes. A difference current I3 between the two circuit branches causes a voltage difference between the mutually connected emitters of the differential amplifier transistors Q1, Q2 and the voltage follower transistor Q3. This voltage difference changes with the differential input voltage VID (=VINP−VINN), thereby causing the transistors Q1, Q2, Q3 to operate with different emitter currents. This adds to the non-linear operation of the emitter follower circuit operation. Additionally, the output signal range is limited by the available range of output voltage across the load resistance RL.